For internal combustion engines Homogenous Charge Compression Ignition (HCCI) has a big potential in reducing the fuel consumption. In vehicles, compared to spark ignition (SI) operation, a fuel reduction of about 20% could be achieved if it would be possible to run the engine in HCCI throughout the operating range normally used in a vehicle, as defined in the EU test cycle for cars.
In HCCI, the combustion mixture may have a relatively low fuel content. The reason is that the maximum temperature during combustion has to stay within a range. The lower end of the temperature range may be set by the minimum temperature needed for compression ignition. The upper end of the range may be set to limit nitrogen oxide (NOx) formation. The low fuel content can be achieved by high air content or by re-circulating exhaust gases. However, the low fuel content may restrict the maximum torque of the engine, which means that during normal operation, a large number of mode switches between HCCI and SI may be performed. In addition, HCCI is also restricted from operation in low torque operating regions since the high air content or re-circulated exhaust gases results in the temperature being too low for compression ignition to take place.
A number of suggestions have been presented to extend the torque range of the HCCI operation. So called boosted HCCI utilizes a special compressor for increasing the maximum torque limit, but results in a complicated and costly solution. Another suggested solution is a negative overlap of exhaust and inlet valves, but this requires extra fuel injection to control combustion which leads to increased wall heat losses, which in turn deteriorates fuel efficiency.
U.S. Pat. No. 6,877,464 B2 describes an internal combustion engine with intercylinder gas channels between pairs of cylinders with overlapping exhaust and induction strokes, so that, during one operational mode, burned gas from a preceding cylinder can be introduced in a following cylinder. Thereby, spark ignition takes place in the preceding cylinder and compression ignition takes place in the following cylinder.
A disadvantage with this solution is that the intercylinder gas channels are controlled by valves sharing camshafts with ordinary inlet and exhaust valves, which limits the possibility to regulate, for example by variable valve timing, the intercylinder gas channel timing without affecting the timing of ordinary inlet and exhaust valves. Another disadvantage is that only half of the cylinders can operate in a HCCI mode, which limits the fuel consumption reduction allowed by HCCI.
Besides in HCCI, in normal SI Otto cycles and in diesel operation, exhaust gas recirculation (EGR) may be used in order to reduce nitrogen oxide (NOx) emissions. However, specifically during diesel operation, known solutions may limit the amount of exhaust gases introduced into the cylinders, so that it is less than desired.